At the present time, agility requirements are increasingly exacting, notably because of the increased use of the electromagnetic spectrum, the systematic increase in required bandwidths and the software reconfigurability of mobile terminals.
An RF transmitter, as illustrated in FIG. 1, generally comprises three main units between, for example, the generation of a high-power radio signal and the antenna.
These three units are the power amplifier PA having an output impedance Zout; the matching network CA, at the output of the amplifier, which ensures the transfer of energy from the amplifier to the antenna; and the antenna filter FA, which ensures the spectral purity of the signal delivered by the system to the antenna ANT.
Since the amplifiers, filters and antennas are often made by different suppliers, the components must therefore operate at a given characteristic impedance Zc.
With advanced miniaturization of mobile telephones, antenna manufacturers would like to be freed from the constraint of a standard impedance for reasons of optimization. As a result, system designers must either add a matching network at the output of the filter or provide filters having an impedance Zx which matches the specific impedance of the antenna, they thus tend to implement arrangements such as that illustrated in FIG. 2.
Compensating for these dynamic behaviours not only ensures, in the short-term, a reduction in the power consumed by current systems but also enables use of certain systems that are innovative because of their reconfigurability.
Several prior-art approaches are described in the literature and notably in the following article: “An Automatic Antenna Tuning System using only RF-Signal Amplitudes”, E. L. Firrao, A. J. Annema and B. Nauta, IEEE TCAS-II, 2008.
The authors, from the University of Twente and from Thales, propose dynamic antenna matching combined with the concept of matching by separately adjusting the real part and the imaginary part of the impedance.
After the system has sensed the mismatch between the antenna and the RF units, dynamically adjustable elements make it possible to change the impedance locus of the antenna. The impedance is matched after a convergence time. The principle was demonstrated for a 900 MHz signal, and for discrete impedances. However, effectiveness over a wide frequency band, and for far-reaching impedance loci, is as yet unproven.
Other authors, including a team at the CEA, have developed a substantially different approach in collaboration with the University of Bordeaux and the Sorin Group (which has notably specialised in the development of pacemakers), which approach is described in the article: “A fast and accurate automatic matching network designed for ultra low power medical applications”, Wai Chan; De Foucauld, E.; Vincent, P.; Hameau, F.; Morche, D.; Delaveaud, C.; Dal Molin, R.; Pons, P.; Pierquin, R.; Kerherve, E.; Circuits and Systems, 2009, IEEE ISCAS 2009.
A matching network comprising several variable elements is controlled by a microcontroller or processor, the role of which is to specify control set points depending on analysis of a signal sampled via an attenuator.
This approach was demonstrated around the 402-405 MHz ISM band and assumes a processing time of almost 1 ms. FIG. 3 illustrates automatic antenna impedance matching at 400 MHz using a microcontroller.
Comparable results have been published, by the same team, for the 2.4 GHz ISM band, in the following article: “A 2.4 GHz CMOS automatic matching network design for pacemaker applications”; Chan Wai Po, F.; De Foucauld, E.; Vincent, P.; Hameau, F.; Kerherve, E.; Molin, R. D.; Pons, P.; Pierquin, R.; Circuits and Systems and TAISA Conference, 2009.
Although the theoretical principle of direct matching using a microcontroller has been demonstrated, and was subject matter for a patent filed by the CEA (EP 2 037 576), its use remains complicated and limited by the physical implementation. Thus, impedance coverage depends on the complexity of the matching network and notably on the number of adjustable elements and their nature (inductors or capacitors). Moreover, this approach, which has the benefit of being direct (and non-iterative) and is therefore likely to be fast, requires that it be possible to calculate analytically the value of the adjustable elements. The analytical calculation is easily implemented within the microcontroller; however, the complexity of the implementation increases with the number of variable elements and with the presence of filtering elements between the amplifier and the antenna.
The effectiveness of this approach depends on the number of tunable elements available, on their range of variation, on their quality and on the extent to which the impedance of the antenna varies.
Thus, if a post-amplification filter is required, the arrangement illustrated in FIG. 4 may be employed. A sensor C is placed at the output of the amplifier and has an input impedance and an output impedance Z1. The series of units furthermore comprises the filter F and a matching network CA allowing conversion from the impedance Z1 to an impedance Z2, a processor P being provided to control the matching network and to adjust the latter depending on variations in the impedance of the antenna ANT.
Matching takes place after the filter, which must then have a characteristic impedance identical to the output impedance of the power amplifier. It is recalled that filters are reciprocal elements, i.e. they have the same input and output impedance.